Forms of Combination of Sulfur in Vulcanization of Rubber

Abstract A certain portion of the sulfur used appears as polysulfide sulfur in all vulcanization processes. The polysulfide concentration passes through a maximum with time, so that it must be considered to be an intermediate product which disappears to form final products. It is shown that the dependence of polysulfide sulfur concentration on time may be described by an empirical equation with three constants, of which one is related to the system, the other two depending in a characteristic way on concentration or temperature. The application of this equation permits recognition of a close relation between the kinetics of sulfur decrease and that of polysulfide formation. This leads to a kinetic theory of formation and decrease of polysulfide sulfur which is developed in detail. Applying the experimentally determined first order velocity constants of sulfur decrease and conditions obtaining for polysulfide maximum, velocity constants for polysulfide decrease can be calculated. With these, equations for the functional relation between polysulfide sulfur and time as well as for the increase of final products may be set up. Good correspondence between theory and experiment is found for the dependence of polysulfide sulfur in the vulcanizate on time. Comparisons were examined which indicate the validity and applicability of the kinetics developed.